Conventionally, in an image forming apparatus by an electrophotographic method, the surface of a photoconductor drum is charged and exposed to form an electrostatic latent image, the electrostatic latent image is developed with toner to form a toner image, the toner image is primarily transferred onto an intermediate transfer belt to form a composite toner image, the composite toner image is then secondarily transferred onto a recording medium, and then the composite toner image on the recording medium is fixed. In such an image forming apparatus, an image adjustment pattern such as adhesion amount control pattern, color matching pattern and gray level correction pattern is formed on the intermediate transfer belt as a toner image, and the image adjustment pattern is detected to perform an image adjustment.
As the image adjustment pattern on the intermediate transfer belt passes through a secondary transfer roller, toner adheres to the secondary transfer roller, so that the toner then adheres to the rear face of a paper sheet on which next image formation is to be performed, with a result of sheet stains. Accordingly, during the image adjustment, generally, the secondary transfer roller is released away from the intermediate transfer belt so as to prevent the toner from adhering to the secondary transfer roller. However, with this constitution, when the secondary transfer roller is brought into contact with the intermediate transfer belt after image adjustment, the intermediate transfer belt vibrates and changes in speed, with the result that color misregistration occurs during image formation irrespectively of the image adjustment having been executed. Due to this, image formation is started after the secondary transfer roller comes into complete contact with the intermediate transfer belt. However, this causes a large time lag from when a print button is pushed until when image formation is started, resulting in deterioration of printing productivity as a problem. Moreover, because of a difference between a speed of the intermediate transfer belt at the time of image adjustment when the secondary transfer roller is separated from the intermediate transfer belt and a speed of the intermediate transfer belt at the time of image formation when the secondary transfer roller is in contact with the intermediate transfer belt, even if the image adjustment pattern is detected with high precision, there occurs a change in image formation position at the time of actual image formation, which causes a problem that misregistration of color may occur.
As a solution, there has been proposed an image forming apparatus in which a secondary transfer roller is constantly in contact with an intermediate transfer belt. In this image forming apparatus, staining of the secondary transfer roller are prevented, for example, by providing a cleaning member for cleaning toner adhering onto the secondary transfer roller during the formation of the image adjustment pattern or by applying a bias voltage of same polarity as the toner to the secondary transfer roller.
However, with the cleaning member provided on the secondary transfer roller, not only such a cleaning member as a blade but also a container for supporting the cleaning member and containing toner removed by the cleaning member as well as a mechanism for discharging toner are necessitated, leading to a complicated, larger-scale apparatus. Besides, there is a need for enhancing the releasability for toner of the secondary transfer roller, posing a problem of a cost increase involved.
With the constitution that a bias voltage of same polarity as toner is applied to the secondary transfer roller, since the adhesion amount control pattern or the color matching pattern involves formation of a toner image extending over a relatively long distance on the intermediate transfer belt, it could occur that toner adheres to the secondary transfer roller even with suppression of the toner adhesion by the bias voltage. Thus, there is a need for cleaning the secondary transfer roller with a cleaning member before image formation, posing a problem of deteriorated print productivity.
Patent Document 1 proposes that a transparent toner layer is formed at a ground layer or top layer of the colored toner image in the primary transfer so as to prevent staining of the rear face of the recording medium. However, this proposal involves a transparent toner cartridge in addition to a colored toner cartridge, posing a problem of an increase in the apparatus scale.